The project is a continuing study of the regulation of oxytocin release. In conscious goats, physiological levels of progesterone and estrogen modify the amount of oxytocin released in response to vaginal distention. A model describing how these sex steroids control the oxytocin-releasing reflex has emerged from studies during the estrous cycle and during anestrus and from an examination of the effects of ovariectomy and steroid replacement therapies on the reflex. The predictive value of this model will be further tested by measuring the responsivity of the reflex in sheep bearing utero-ovarian transplants in the neck. In these preparations, ovarian steroid release into the general circulation can be controlled almost at will by both chemical and mechanical means. To some extent, at least, the sex steroids control reflex release of oxytocin by acting in the brain. Indeed, the inhibitory effect of progesterone is clearly mediated centrally. Thus, the neural mechanism mediating oxytocin release may be analogous to mechanisms effecting the secretion of releasing factors. If this is so, certain biogenic amines, now understood to serve as neurotransmitters in the neural system controlling secretion of releasing factors, may play a similar role in the oxytocin-releasing system. This possibility will be examined. These studies, aimed at a thorough understanding of the effects of ovarian steroids on hypothalamically mediated release of oxytocin, are important because they represent an examination of a steroid-sensitive function of the brain. Their peculiar significance in this regard, however, obtains because the oxytocin-releasing function is particuarly accessible to examination under physiological conditions.